From the Sigil blog:
Basically, one year go the there were no outside contributors. So for one year not much has happened. A small maintenance release happened. Bug fixes were committed but few and far between. Nothing major feature wise.
Now that all changed about a month ago when Kevin Hendricks decided to invest his time in some bug fixes. Then he decided to start working on a plugin framework for Sigil. He's been spearheading the effort to get this feature implemented. It's not ready yet but it's coming a long nicely.